It is well known that the types and amounts of substances present in engine exhaust is greatly affected by the ratio of air to fuel in the mixture supplied to the engine. Rich mixtures tend to produce high amounts of hydrocarbons and carbon monoxide, whereas lean mixtures tend to produce greater amounts of oxides of nitrogen. It is well known that exhaust gases can be catalytically treated to reduce the amounts of these undesirable components and that the minimization of these undesirable exhaust constituents can be achieved with a single catalytic device provided that the air-fuel mixture supplied to the three-way catalytic converter is maintained within a narrow range at stoichiometry, the so-called "converter window".
It has been suggested that a closed loop fuel control system, in which the air-fuel ratio of the mixture supplied to the engine is controlled by a feedback signal from a zirconia sensor exposed to exhaust gases, can maintain the gases supplied to the catalytic converter within the converter window. However, the design of such a control system must meet a number of requirements. The system must be quick reacting in response to changing engine operating parameters, while at the same time must be stable so that the controlled air fuel mixture spends less time out of the converter window. A number of closed loop fuel control systems have been proposed, but none are completely satisfactory. Most use a zirconia sensor exposed to engine exhaust upstream from the converter and use proportional and integral control in the feedback loop. Such systems do maintain some control over the engine operating point but tend to drift out of the converter window over time as a result of changing engine operating parameters.